package Tree;

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

public class 深度优先遍历_迭代 {
    /**
     * 前序遍历顺序：中-左-右，入栈顺序：中-右-左
     *
     * @param root
     * @return
     */
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) return result;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            result.add(node.val);
            if (node.right != null) stack.push(node.right);
            if (node.left != null) stack.push(node.left);
        }
        return result;
    }

    /**
     * 中序遍历顺序：左-中-右，入栈顺序：左-右
     *
     * @param root
     * @return
     */
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) return result;
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()) {
            if (cur != null) {
                stack.push(cur);
                cur = cur.left;
            } else {
                cur = stack.pop();
                result.add(cur.val);
                cur = cur.right;
            }
        }
        return result;
    }

    /**
     * 后序遍历顺序：左-右-中，入栈顺序：中-左-右 出栈顺序：中-右-左，最后翻转结果
     *
     * @param root
     * @return
     */
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) return result;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            result.add(node.val);
            if (node.left != null) stack.push(node.left);
            if (node.right != null) stack.push(node.right);
        }
        return result.reversed();
    }
}
